1. Field of the Invention
This invention relates to spark plugs for use with internal combustion engines, and more particularly to a resettable pressure relieving spark plug for venting the combustion chamber of an internal combustion engine in response to abnormal engine operating conditions.
2. Description of Related Art
The many uses of internal combustion engines are well known. Internal combustion engines extract work from the combustion of a mixture of fuel and oxygen. Combustion typically occurs within the confines of a cylinder wherein useful work is extracted from the expansion of the products of combustion pushing against a movable piston. In other internal combustion engines, such as a rotary or Wankel engine, work is extracted from the expansion of the products of combustion by a rotating rotor. Accordingly, in a conventional piston engine work is extracted by compressing a mixture of fuel and air and igniting the mixture, typically by use of a spark plug, thereby increasing the pressure and temperature of the products of combustion within the confines of the cylinder, and extracting work from the pressure differential between the cylinder and atmosphere. Internal combustion engines are widely used in all modes of transportation.
Internal combustion engines extract work from the pressure generated by the combustion of a volatile fuel/air mixture within a combustion chamber. In a piston engine, a combustion chamber is defined by a pressure vessel formed by the cylinder and a piston slidably disposed therein. Peak cylinder pressures typically fall within the range of 7 MPa to 8 MPa (roughly 1000 psi-1200 psi), and engine components may fail if pressures exceed 15 MPa (2200 psi). Spark-ignition engines typically have compression ratios between 6:1 and 12:1, use carburetors, or fuel-injection systems, and operate on the Otto cycle. Compression-ignition engines use liquid fuels of low volatility, such as fuel oil, have compression ratios between 11.5:1 and 22:1, and operate on the diesel cycle. The four-stroke-cycle engine requires four piston strokes or two crankshaft revolutions per cycle. The two-stroke-cycle engine requires two piston strokes, or only one crankshaft revolution for each cycle.
In the United States, the automobile is the dominant mode of transportation. Each year approximately 2 trillion passenger miles are traveled by car. Accordingly, internal combustion engines are an integral part of every day life. Therefore, maintaining internal combustion engines in good operating condition is an important part of every day life. Internal combustion engines may be damaged, however, when pressure within the combustion chamber (e.g. cylinder) exceeds the maximum design pressure, thereby resulting in engine damage. Such excessive pressure may be caused by any one of several operational occurrences such as hydrolock or detonation.
Hydrolock, also referred to as hydrostatic lock or hydraulic lock, is the phenomenon of engine damage from excessive cylinder pressure due to the presence of an incompressible liquid, typically water, in the engine cylinder. All four-stroke-cycle spark ignition and compression ignition (Diesel) engines, as well as their two-stroke-cycle counterparts, are all at risk of damage from hydrolock, although differences in tolerance and resistance to hydrolock damage exist between engine types. Water may find its way into the engine cylinder in any number of ways. For example, water may enter through a faulty head gasket, or from a leak in the cooling system in the case of water cooled engines. The most common water entry pathway, however, is through the air induction system. The air induction system consists of the airbox, filter, airflow sensor or carburetor, and intake manifold.
In four-stroke-cycle internal combustion engines, hydrolock causes damage when water passes through the engine's air induction system and enters the combustion chamber or cylinder during the intake stroke of the cycle. During the next compression portion of the cycle, the presence of water effectively reduces the volume of the cylinder chamber thereby causing a substantial increase in cylinder pressure well above the design operating pressure due to the incompressibility of water.
Single cylinder, two-stroke-cycle internal combustion engines are less prone to damage from hydrolock since the fuel-air mixture first enters the crankcase prior to entering the combustion chamber. The compression ratio of the crankcase chamber is far lower than the compression ratio in the cylinder. This permits a comparatively large volume of water to enter a two-cycle engine without causing immediate damage and most likely preventing the delivered fuel charge from firing thus stopping the engine before damage could occur. Multi-cylinder, two-stroke-cycle internal combustion engines, however, are prone to hydrolock damage since water which enters the crankcase of one cylinder can be pumped into the non-firing cylinder, since the remaining cylinders continue to operate, thereby causing hydrolock. Accordingly, hydrolock is a significant problem with multi-cylinder, two stroke engines, particularly outboard engines used in marine propulsion.
Automotive engineers have attempted to prevent hydrolock damage by designing air induction systems intended to avoid water ingestion. For example, some manufacturers of light trucks and military vehicles incorporate air intakes which are positioned high on the vehicle. Most vehicles, however, have air induction systems with the air intake located under the hood. For example, vehicles with fuel injected engines are more susceptible to hydrolock because the air intake is typically positioned low, away from engine compartment heat, beneath the hood, such as in the front fender. Accordingly, a vehicle having an air intake positioned below the hood is in danger of ingesting standing water. Even water of a lesser depth may be splashed up into the air intake by movement of the vehicle. Furthermore, off road vehicles, such as modified light trucks, boats, and all terrain vehicles ("ATV's") are presented with an even greater risk of ingesting water due to the presence of water in the off road environment.
Engine damage resulting from the untimely and spontaneous detonation of the fuel/air mixture is a further problem experienced with internal combustion engines. The effects of detonation range from annoying sounds, commonly referred to as engine knock, ping, or preignition, to catastrophic engine failure. Knock sensors and oxygen sensors are two devices frequently employed to abate detonation. Knock sensors detect audible shock waves in the engine block to selectively retard ignition timing, while oxygen sensors monitor exhaust gas oxygen concentrations to detect and correct lean (detonation prone) fuel mixtures.
Detonation, however, remains responsible for a substantial amount of engine damage. A proper combustion event is characterized by a flame front propagating hemispherically away from the ignition source (the spark). As the flame front propagates, it produces a continuing increase in cylinder pressure, effectively driving the piston downward and producing torque on the crankshaft. Detonation, however, is a combustion event wherein the fuel/air mixture spontaneously combusts generating a nearly instantaneous shock (pressure) wave throughout the cylinder in lieu of the propagating front characteristic of a proper combustion event. The nearly instantaneous shock waves characteristic of detonation are detrimental to engine components.
Numerous unsuccessful attempts have been made to provide some way to at least partially alleviate the undesirably high combustion chamber pressures which accompany hydrolock and detonation.
U.S. Pat. No. 4,326,145, issued to Foster et al., discloses a spark plug and a pressure relief adapter for venting a portion of the gases in an engine cylinder during the starting operation.
U.S. Pat. No. 4,699,096, issued to Phillips, discloses a detonation prevention means for an internal combustion engine including a valve which opens in response to a predetermined pressure.
The devices of the background art, however, have only limited venting capabilities and are not suitable for creating a vent passage of sufficient size and venting capacity to effectively expel a sufficient volume of water from a cylinder under hydrolock conditions to prevent damage. Furthermore, the devices of the background art, often require engine modification or the use of complicated and unreliable spark plug assemblies, and, thus, have not gained widespread acceptance. In addition, the background art does not disclose a spark plug for venting cylinder overpressure in response to excessive engine operating temperatures.
Accordingly, there exists a need for an effective pressure relieving spark plug for preventing engine damage resulting from hydrolock and detonation. The references of the background art fail to address the need by providing a resettable pressure relieving spark plug capable of staged venting of a combustion chamber wherein venting capacity is sufficient to prevent hydrolock damage by expelling a sufficient volume of water. In addition, there exists a need for a pressure relieving spark plug which is thermally responsive to excessive engine operating pressure.
My co-pending U.S. Patent application discloses a permanently deforming pressure relieving spark plug which is designed for multiple stage release wherein venting is accomplished by designed structural failure. However, there still exists a need for a resettable (i.e. non-permanently deforming) pressure relieving spark plug that avoids the disadvantages present in the devices of the background art.